Pessel



Feb. 7, 1956 L. PESSEL 2,733,774

UNDERWATER ACOUSTIC DIAPHRAGM Filed Feb. 28. 1950 lNV NTOR BY 1.20am:Pain.

ATTORNEY United States Patent UNDERWATER ACOUSTIC DIAPHRAGM LeopoldPessel, Whitemarsll, Pa., assignor, by mesh-e assignments, to the UnitedStates of America as represented by the Secretary of the NavyApplication February 28, 1950, Serial No. 146,651

3 Claims. (Cl. ISL-32) This invention relates to improved materials formaking acoustic diaphragms and, more particularly, to acoustic materialsintended to be used submerged in water.

Loudspeakers which are placed on the decks of submarines are necessarilyexposed to water for extended periods of time whenever the submarinesubmerges. In the past, various types of loudspeakers have been used inthis type of service, including cone-type speakers. All parts of theloudspeaker must be made of waterproof material. Waterproof materialssuitable for loudspeaker cones have formerly been known and used. Thesecones, however, have been found to suffer from disadvantages in battleuse. It has been found that ordinary waterproof speaker cones are oftenpunctured by nearby, underwater explosions when the cone is submerged. Acone which has thus been punctured is useless for further service andmust be replaced.

The present invention relates to an acoustic material comprising a webof flexible, fibrous material impregnated with a water-insoluble resinand having a surface coating of a water-insoluble, hydrophilic,film-forming substance. The invention also includes acoustic diaphragrnsmade of this improved material.

One object of the present invention is to provide an improved waterproofmaterial suitable for loudspeaker cones intended for underwater use.

Another object of the invention is to provide an improved waterproofacoustic diaphragm.

Another object of the invention is to provide an improved loudspeakercone for underwater use.

Another object of the invention is to provide an improved loudspeakercone which will resist the effects of underwater explosions.

These and other objects will be more apparent and the invention will bemore readily understood from the following detailed description,including the drawings, of which:

Fig. 1 is a plan view of one form of acoustic diaphragm made of theimproved material of the present invention, and

Fig. 2 is a cross section view taken along the line 22 of Fig. 1.

Acoustic diaphragrns, such as used in loudspeaker cones, are usuallymade of a thin, flexible web of molded, felted, fibrous, cellulosicmaterial. Small percentages of resinous binders may be mixed with thefibrous material. The cellulosic material is usually a good grade ofwood pulp, which may be combined with various amounts of less expensivefibers, such as waste paper or textile fibers. Acoustic diaphragms mayalso be molded out of impregnated textile fabrics or even fine mesh wirescreen material. If the diaphragm is to be exposed to water or if itmust be submerged in water, the fibrous base ma.- terial is usuallyimpregnated with relatively high percentages of waterproof binder. Thewaterproof binder is usually a water-insoluble, thermosetting resin. Anywater-insoluble, thermosetting, resin may be used. Preferred examplesare condensation products such as urea ice formaldehyde resins, phenolformaldehyde resins, melamine formaldehyde resins, and phenol furfuralresins. These and other thermosetting resins generally havewaterrepellent, or hydrophobic, properties.

It has been found, as previously mentioned, that speaker cones made offibrous materials impregnated with these resins are subject to punctureif used in places subject to underwater explosions. The presentinvention is a result of the discovery that rupturing of the cones byexplosions is due to entrapment of air bubbles on the surface of thecone. Air bubbles are trapped due to the hydrophobic properties of thewaterproof resin binder. Apparently, when an explosion occurs nearby inthe water, any entrapped air bubbles upon the surface of the conerapidly contract and deliver a hammer-like blow to the surface of thecone over a small area. The blow often has suflicient force to cause apuncture. If an attempt is made to make the diaphragm out of materialstrong enough to withstand the puncturing force, the cone usually doesnot have sufficiently good acoustic response.

It has been found that the waterproof properties may be retained and, atthe same time, the entrapment of air bubbles prevented by providing thesurface of a waterproofed speaker cone with a coating of awater-insoluble, hydrophilic material. This is illustrated in thedrawing wherein a typical loudspeaker cone 2, having an outer surface 4and an inner surface 6, is provided with a coating 8 on the outersurface and a similar coating 10 on the inner surface.

The coating must be a water-insoluble, hydrophilic substance whichadheres well to the base material. A hydrophilic substance is one whichis readily wetted by water. That is, if a spray of water is directed toa surface composed of the substance, it will form a uniform film overthe surface instead of gathering in droplets, as would be the case witha hydrophobic material.

A preferred coating substance for the present invention comprises aninsolubilized and partially hydrolized polyvinyl alcohol. A suitableinsolubilized polyvinyl alcohol can be made by first saponifying about50% f the acetate groups of polyvinyl acetate. This polyvinyl alcohol isthen made increasingly water resistant by giving it an insolubilizingtreatment by any one of a number of well known methods. For example, thepartially hydroiized polyvinyl alcohol may be acidified withhydrochloric acid to a pH of 4-5. A coating of this material may then beapplied to the diaphragm by any convenient method, such as by dipping,spreading, or brushing. After draining off the excess solution, thediaphragm is heated in an oven for about 30 minutes at C. The resultantsurface is both waterproof and also suficiently hydrophilic to preventtrapping of air bubbles when the diaphragm is under Water.

The partially hydrolized polyvinyl alcohol may also be insolubilized byheating it with a small proportion of dimethylol urea, using ammoniumchloride as a catalyst. An acoustic diaphragm may be provided with acoating of the material by, first, mixing the three ingredients,applying the mixture to the surface of the diaphragm, and heating for atleast 15 minutes at 150 C. More specifically, the above solution may bemade by first dissolving 10 parts by weight of the partially hydrolizedpolyvinyl alcohol, previously referred to, in a mixture composed of 100parts by weight of any suitable solvent such as ethanol or propanol andparts by weight of water. A second solution is made up by dissolving 5parts by weight of dimethylol urea in 100 parts by weight of water. Athird solution is made up by dissolving 5 parts by weight of ammoniumchloride in 100 parts by weight of water. The coating mixture is made bymixing 5 parts by volume of the polyvinyl alcohol solution with 1 partby volume of the dimethylol urea solution, 1 part by volume spasms ofthe ammonium chloride solution, 10 parts by volume of water, and 12parts by volume of ethanol or propanol. In a preferred form of carryingout the invention, a speaker cone composed of a cloth impregnated withphenol formaldehyde resin was treated with two coats of the abovedescribed solution. Each coat was permitted to dry in air and then eachwas heated for 15 minutes at 150 C. The cone was then further treatedwith a solution comprising parts by volume of the ethanol or propanolsolution of polyvinyl alcohol, described above, 15 parts by volume ofalcohol (such as ethanol or propanol) parts by volume of water and 0.5part by volume of 10% hydrochloric acid. After treating with thissolution, applied as a coating, the diaphragm was again permitted to airdry and then heated for minutes at 150 C.

It was also found that the hydrophilic properties of the cones made asabove described could be improved somewhat by scrubbing them in Waterwith a soft bristle brush until the surface became uniformly wetted bythe water.

In general, the polyvinyl alcohol should be hydrolized to the extentthat it is still water-wettable. The partially hydrolized materialshould then be insolubilized just so that it is substantially insolublein water. Any polyvinyl alcohol meeting these two requirements can beused in the present invention.

Although water-wettable, water-insoluble, polyvinyl alcohol is thepreferred coating material for use in the invention, other materials canalso be used. A film of rubber latex may be used, for example. This maybe applied to the surfaces of a resin-impregnated acoustic diaphragm,either from aqueous emulsion or organic solvent solution. The volatilecomponents are permitted to evaporate. An example of a suitable rubberlatex is a copolymer of butadiene and acrylonitrile. Any of thecommercially available copolymers may be used, such as Hycar OR-15 orHycar OR25. Copolymers of butadiene and styrene may also be used. Anexample of this type of copolymer is Hycar OS10. The latex films do notadhere firmly to all types of resinous surfaces, however. In general,the rubber latex which is used may be any synthetic rubber havinghydrophilic properties. Natural rubber latex may also be used.

Another example of a film-forming substance which can be used as thewater-wettable coating of the present invention is an emulsion of apolyvinyl chloride. Various wetting agents may be incorporated with theemulsion to improve its spreading properties when it is applied to thediaphragm. Suitable wetting agents include isopropyl naphthalene sodiumsulfonate, dioctyl ester of sodium sulfosuccinic acid, sodiumtetrahydronaphthalene sulfonate, and cetyl dimethyl benzyl ammoniumchloride. Any other compatible wetting agent may also be used. As anexample of use, an aqueous emulsion of polyvinyl chloride was preparedhaving 56.5% solids content by weight and containing 1.5% by weightwetting agent. A thin coating of this composition was applied to bothsurfaces of a waterproof speaker cone and the cone was then heated at150 C. for 15 minutes. The wetting agent can be omitted if desired. Itdoes not affect the hydrophilic properties of the dried film to anyextent.

Still another example of a hydrophilic, film-forming, Water-insolublcsubstance which can be used as the surface coating in the materials ofthe present invention is hardened gelatin. The gelatin can be applied asa water sol and hardened by any wellknown method such as soaking in asolution of chrome alum, or a dichrornate. Although resistant to theswelling action of water, the hardened, insoluble gelatin ishydrophilic.

There has thus been described an improved material having Waterproof andwater-wettable properties. Although the material is suitable for use inany application requiring a water-wettable but water-insoluble surface,it is particularly adapted to be used as the acoustic diaphragm in aloudspeaker which is submerged in water at least a part of the time.There has also thus been described an improved acoustic diaphragm forunderwater use and not readily subject to rupture due to nearbyunderwater explosions.

l claim as my invention:

1. in underwater sound equipment, an acoustic diaphragm capable ofresisting the shock of nearby underwater explosions, said diaphragmcomprising a molded web of porous, flexible fabric impregnated with awaterinsoluble, hydrophobic, thermosetting resin and having an exposedhydrophilic coating comprising a partially hydrolized, insolubilizedpolyvinyl acetate.

2. in underwater sound equipment, a loudspeaker diaphragm including aneck portion, a flanged rim and a main body portion of conical shape,said diaphragm comprising a molded web of porous flexible, fibrousmaterial impregnated with a water-insoluble, hydrophobic, thermosettingresin and having an exposed surface coating consisting essentially of awater-insoluble, hydrophilic, film-forming, resinous material.

3. 1n underwater sound equipment, a loudspeaker diaphragm including aneck portion, a flanged rim and a main body portion of conical shape,said diaphragm comprising a web or" flexible, porous, fibrous materialimpregnated with a water-insoluble, hydrophobic, thermosetting resin andhaving an exposed hydrophilic surface coating comprising a substanceselected from the class consisting of partially hydrolized andinsolubilized polyvinyl acetate, natural rubber latex, copolymers ofbutadicne and acrylonitrile, copolymers of butadiene and styrene,polyvinyl chloride and hardened gelatin.

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